Field of the Invention
The present invention relates to a liquid crystal display device. The present invention relates to a liquid crystal display device having a light source unit, a liquid crystal cell, a polarizer and a protective film in detail.
Description of the Related Art
Recently, use of Liquid Crystal Display, LCD, is enlarged and it is used for a TV, a monitor, a smart phone, a car navigation system, a digital camera, a digital signage and etc.
A liquid crystal display device generally has polarizing plates on both a liquid crystal cell's viewing-side and a light source unit-side. A polarizing plate used for a liquid crystal display device generally has a configuration formed by bonding a transparent protective film on both sides of a polarizer which is composed of polyvinyl alcohol-based film or the like adsorbed and oriented with iodine or a dichroic dye. For the protective film, generally a protective film of a cellulose acylate-base typified by cellulose acetate has been widely used since it has high transparency and can easily be ensured adhesion between the polyvinyl alcohol film used in the polarizer.
Recently, as expanding applications of a liquid crystal display device, a large-size and high-quality texture is needed for a liquid crystal display device. To reduce the mass of the large-sized liquid crystal display device, the thickness of the glass substrates are becoming thinner from traditional 0.7 mm to 0.5 mm or less. Along with it, after the liquid crystal display device is preserved under a high-temperature and high-humidity environment and then light the liquid crystal display device, there are problem cases of occurring a bending in the liquid crystal cell and further occurring a display nonuniformity (See JP-A-2012-137723).
In JP-A-2012-137723, it is described that using a film having low moisture permeability such as polyethylene terephthalate film makes it possible to be prevented from occurring from bending of the liquid crystal cell where it has been preserved under a high temperature and high humidity environment and then is lighted and to be prevented from occurring a display nonuniformity.
On the other hand, in case where using a polyester film having a larger birefringence than the cellulose acylate-base film as a protective film, it is known that a rainbow-like color unevenness (rainbow-like unevenness) occurs and it is a problem that impairs visibility (See JP-B-4962661 and JP-A-2012-256014).
To solve this problem, JP-B-4962661 and JP-A-2012-256014 disclose a liquid crystal display device having a light source unit which emission intensity spectrum is sequentially such as a white color light-emitting diode (white color LED) and in-plane direction retardation (hereinafter referred to Re) is sufficiently large value. The white color light-emitting diode in JP-B-4962661 and JP-A-2012-256014 is a phosphor type element, that is, an element which emits white color light by combining a light emitting diode emitting blue light or ultraviolet rays by using a compound semiconductor and a phosphor. By such a configuration, even light emitted from the liquid crystal display device is different transmittance for each wavelength, various wavelengths light of red color, green color and blue color constituting the light having a continuous emission intensity spectrum emitted from white color LED is transmitted through the protective film. Therefore, light passing through the protective film is observed white as a whole and so the rainbow-like unevenness is not visually recognized. In addition, in the case where the liquid crystal display device having a protective film having large Re is observed from an oblique direction rather than from the front direction, there is a case where apparent retardation Re of the protective film become a small value as compared to when observing the liquid crystal display device from the front direction and so the rainbow-like unevenness is visually recognized. In JP-B-4962661 and JP-A-2012-256014, an effective range of a thickness direction retardation (hereinafter referred to Rth) is determined for the protective film keeping an apparent large Re in case where the liquid crystal display device is observed from an oblique direction.